US3542640A - Method for drying a wet foam containing cellulosic fibers - Google Patents

Description

NOV. 24, 1970 "N, 9,.F I ETAL 3,542,640

7 METHOD FOR DRYING A WET FOAM CONTAINING CELLULOSIC FIBERS Filed March 23, 1967 INVENTORS Norman D. Friedberg Frank S. Adams BY M J, M

N v Q o V N 3 mm Gm d j mm 8 ATTORNEY United States Patent 3,542,640 METHOD FOR DRYING A WET FOAM CON- TAINING CELLULOSIC FIBERS Norman D. Friedberg, Cincinnati, and Frank S. Adams, Wyoming, Ohio, assignors to The Procter & Gamble Company, Cincinnati, Ohio, a corporation of Ohio Filed Mar. 23, 1967, Ser. No. 625,367 Int. Cl. D21d 3/00 U.S. Cl. 162---101 4 Claims ABSTRACT OF THE DISCLOSURE An improved method for drying a wet foam containing randomly dispersed fibers by drying the wet foam in a series of steps to produce a low density, absorbent, fibrous paper or sponge-like material. The foam is deposited as a uniform sheet on a movingforaminous support where most of the water is removed by drainage. The remaining water in the foam is removed by phase change. Significant improvement in the drying rate may be achieved by heating the foam above room temperature. Water removal by phase change may include one or the other or both of the steps of impinging a hot gas normal to at least one surface of the wet sheet and/or blowing a hot gas through the sheet.

The invention is particularly suited for drying a stable and homogeneous wet foam containing uniformly dispersed fibers. Foams of this kind can be made by an apparatus as described and claimed in the copending application of Norman D. Friedberg and James P. Hutchins, Ser. No. 603,548, filed Dec. 21, 1966, now Pat. No. 3,- 506,538, which application is entitled Apparatus for Producing a Homogeneous Foam.

Low density fibrous products can be produced by drying a foamed slurry containing suspended fibers. The wet foam can be deposited on a foraminous support and dried to produce the finished product. Heretofore, drying of wet foams has been done in two stages whereby most of the water was removed by drainage, including both gravity drainage and vacuum induced forced drainage. In the prior art, the remaining water is removed by phase change which is specifically limited to moving a hot drying gas parallel to the surfaces of the foam sheet. This latter step is defined in the art as parallel flow drying. A drying process of this kind is described in U.S. Pat. 1,870,279, issued to F. L. Bryant on Aug. 9, 1932. One of the major drawbacks to the commercial utilization of the prior art process has been length of time needed to remove the water from the wet foam to produce a dry fibrous product.

It is an object of the present inventiton to provide an improved method of drying a wet foam containing randomly dispersed fibers in a more eflicient way in order to minimize drying time and expense.

Another object of the invention is the provision of an improved method of the above character which will dry the product efficiently without damage to the fibers by the use of a sequence of drying steps arranged for optimum results.

The nature and substance of the invention can be summarized briefly as comprising a method of drying a wet foam containing randomly suspended fibers to produce a low density paper or sponge-like product. The method ICC is carried out by depositing the foam on a foraminous support in the form of a sheet whereupon it is drained to remove from about to about of the water. Drying is completed by phase change of the remaining water, i.e., causing the remaining water to vaporize. In the present invention, the removal of water by phase change is accomplished either by impinging a hot gas normal to at least one of the surfaces of the foam sheet, or it can be carried out by blowing a hot gas through the sheet to remove the water remaining after drainage. In some cases it may be preferred to carry out phase change by a combination of steps including both normal hot gas impingement and blow through. The entire drying process can be accelerated if the foam is initially heated to an elevated temperature.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as forming the present invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawing which illustrates schematically an apparatus that can be employed in carrying out the method of the invention. Parts and percentages herein are by weight.

A wet foam containing suspended fibers can be produced by an apparatus of the type described in the aforesaid Friedberg et a1. application. Some elements of this apparatus are shown in the drawing, since it will be helpful in understanding the present invention. Processes for preparing such foams and their utility are also described in U.S. Pat. 1,746,280 issued to F. L. Bryant and in the copending application of Mueller et al., Ser. No. 319,777, filed on Oct. 29, 1963, now Pat. No. 3,311,115.

Referring to the drawing, a wet foam is produced in the foaming vessel 10. The vessel 10 is provided with an inlet 11 for receiving a fibrous slurry .The slurry preferably contains suspended cellulosic fibers. However, the drying method of this invention is not limited by the type of material suspended in the fibrous slurry. The invention can be applied to the drying of any fibrous slurry containing suspended fibers such as synthetic polymers, metal filaments, glass fibers, carbon threads, nylon and any equivalents and/or combinations thereof. In the use of fibers other than those made from natural cellulose, it is necessary to use a binder in the slurry so that the ultimate end product will be cohesively stable.

The vessel 10 is also provided with an inlet 12 for the injection of air. A third inlet 13 is used to introduce a suitable surfactant. A discharge conduit 14 is provided for the withdrawal of wet foam which is ultimately deposited as a uniform thin sheet on a foraminous support 15. Preferably, the foraminous support 15 is a fine mesh wire as commonly used on a Fourdrinier. It will be understood, however, that the foraminous support 15 can, in fact, be any suitable porous surface that will support the foam and permit the practice of the drying process as herein described. A beater (not shown) projects into the vessel 10 to whip air into the slurry to produce a wet foam containing randomly dispersed fibers.

The slurry introduced through the inlet 11 preferably contains cellulosic fibers which range in length from about /2 mm. to about 7 mm. An example of a suitable slurry would be one containing cotton cellulose linters refined to a Canadian Standard freeness of from about 350 ml. to about 500 ml., and preferably in the range of from about 400 ml. to about 450 ml. In the preferred slurry, the fibers are suspended in water, preferably demineralized. The slurry can range from about 1% to about 6% and is preferably about 1.5-% by weight cellulose fibers.

The surfactant introduced at the inlet 13 can be selected from any of the surfactants described in the copending application of Megison and Barton, Ser. No. 545,689, filed Apr. 27, 1966, now Pat No. 3,430,630, in an amount ranging from about 0.05% to about 0.50% of the slurry on an active basis. Preferably, the surfactant is a straight chain alkyl benzene sulfonate having an approximate molecular weight of 342 and a chain length ranging from about C to about C and averaging about C used as the sodium salt. Preferably, the surfactant is prepared as a solution and introduced at a rate of about 1 /2 parts surfactant to 100 parts slurry. The mixture in the foaming vessel 10 is whipped into a foam having a volume up to seven times the original slurry volume. Generally, the foam volume is from about 1.85 to about 4 times the original slurry volume prior to withdrawal through the conduit 15. The slurry volume increase due to foaming is controlled such that the finished product has the characteristics that are being sought.

To improve drying results, the foam can be pre-heated above room temperature (70 F.) to an elevated temperature of about 120 F. in the vessel 10. Preheating of the foam must be limited such that the foam is heated to a temperature within about 50 F. higher than the Wet bulb temperature of the surrounding atmosphere. Undesirable foam expansion and disruption caused by too rapid water vaporization, as well as air expansion within the foam bubbles, will occur if the foam is pre-heated to more than about 50 F. above the wet bulb temperature of the surroundings. Because of the water content of the foam, the Wet bulb temperature in the space immediately surrounding the foam sheet is usually at or very near the dry bulb temperature (room temperature). Thus, the temperature of the preheated foam need only be held to within about 50 F. of room temperature.

It follows, of course, that heating the foam to temperatures substantially above 120 F. should be avoided unless the surrounding temperature is maintained within about 50 F. of the foam temperature. Foam preheating has been found to produce a higher rate of free gravity drainage of water in the first drying stage. The step of foam preheating can be omitted if desired as, for example, when the drying rate for the foam is not particularly critical.

The foam is pumped through the conduit 14 from where it is deposited in the metering trough 15a by means of the oscillating nozzle 16. The foam is deposited on the foraminous support 15 in the form of a sheet of uniform thickness, the latter being controlled by the slice 17.

The foam sheet 18 is carried forward continuously on the tforaminous support or wire 15 through a series of drying steps. These drying steps in a broad sense can be broken down into two phases. The first of these is drying by drainage wherein about 75% to about 85% of the water is removed. Drainage can be accomplished entirely by free gravity drainage. Alternatively, drainage can be carried out in two steps wherein the majority of the water is first removed by free gravity drainage (about to about 65%) and the remainder (about 15% to about 30%) by forced drainage, the combination preferably giving a minimum total time. Forced drainage can be achieved by the application of a low vacuum to the surface of the wet foam sheet, or by lightly pressing a screen against the top surface of the foam, or both. The water remaining in the foam after partial drying by drainage is removed by water phase change in one or two steps, as more fully explained hereinafter.

Foam thickness can be varied from about /2" to about 6", although a thickness of from about to about 2 will be considered typical in the further discussion of the invention. The density of foam is measured as a function of its dried weight, generally ranging from 0.5 to 2.5 lbs. per cubic foot, although it can be as much as 12 lbs. per cubic foot. The preferred wet foam described heretofore would have a dried density of about 2 pounds per cubic foot.

As the support 15 is continuously moved forward, the Wet foam is drained by gravity. Drainage from the foam is collected in the pan 19 mounted beneath the support 15 as shown. If heat has been added to the foam in the foaming vessel 10 such that the initial foam temperature is about 120 F., then about to about 85% of the water from the original wet foam will be removed in this manner. If the foam is gravity drained without preheating, about 50% to about 70% of the water will be removed.

Additional water can be removed by forced drainage, if desired, as by the use of vacuum boxes 20 acting through the lower surface of the support 15. The vacuum acting on the foam must be at least .5 inch of water and must be held to a maximum value no greater than 3 inches of water. Any vacuum greater than 3 inches of water will tend to collapse the foam. Preferably, a vacuum of about 2 inches of water is satisfactory to achieve the objects of the invention. The vacuum effect may be accompanied by light pressing of the top surface of the foam to achieve additional forced drainage. Between 15 and 30% of the original water content is removed by forced drainage regardless of whether or not the foam has been preheated.

After about 7585%, preferably about of the original water has been removed by drainage (which may include gravity drainage alone or gravity drainage plus forced drainage), the remainder is removed by Water phase change (i.e., converting the water from a liquid to a vapor) since the frail nature of the partially dried foam precludes mechanical removal of any additional water. A hot drying gas, preferably hot air, is preferably impinged normal to the upper and lower surfaces of the foamed sheet 18. The hot air is introduced through the inlets 21 and 22 whereupon it enters the plenums 23 and 24. The hot air is introduced through the upper and lower nozzles 25 shown schematically mounted within the blowing chambers 26 and 27. The nozzles 25 impinge the hot air against the sheet in a direction normal to its upper and lower surfaces, whereupon it is deflected so that it flows laterally from the area of impingement. The hot air impingement velocity may increase as the water content of the foam decreases. For example, the sheet entering the chamber 26 may have the jets 25 impinge hot air normal thereto at a velocity of from about 2000 to about 4000 feet per minute. The velocity of the hot gas emited from the nozzles 25 in the chamber 27 can be increased to a range of from about 10,000 to about 12,000 feet per minute. The hot gas velocity can be increased in three or more stages, if desired, such that as the foam becomes dryer and thus stronger, the hot gas velocity is increased thereby accelerating drying without damage to the foam itself. The specific velocities used in any case will depend on several variable factors such as the density of the foam structure, the strength of the foam, the amount of moisture remaining in the foam, etc.

It is also possible to practice the step of hot gas impingement by impinging the hot gas against only one surface of the foam sheet. This will substantially reduce the rate of drying as compared with two surface hot gas impingement. Thus it would find practical application only in those cases where drying rates are not significant.

The hot air is initially introduced at the inlets 21 and 22 at a temperature ranging from about 200 F. to about 400 F. Because of the cooling effect of the water evaporated from the foamed sheet 18, temperatures in this range do not cause any damage to the cellulose material therein.

Some of the hot air from the chambers 26 and 27 with moisture evaporated from the drying foam sheet 18 can be diverted into the chamber 28 surrounding the metering trough a, the pan 19 and the vacuum boxes 20. For best results, it is desirable to control the wet bulb temperature in the atmosphere surrounding the gravity and mechanical drying stages so that it is at least 120 F. but no greater than a maximum of 140 F. This can be accomplished by the use of the chamber 28 and the moist heated air emanating from the blower chambers 26, 27.

The foam sheet -18 may have all of its moisture removed by normal hot gas impingement as just described. If the properties of the foam permit, however, drying by hot gas impingement can be carried out until the foam contains about 1.5% to 15%, preferably 10%, of its original moisture content. The remaining water is then removed by a second phase change step carried out by blowing a hot dry gas, preferably hot air, through the sheet in a direction normal to its broad upper and lower surfaces.

The step of hot air blow through is carried out as the foam sheet 18 passes through the blower chambers 29 and 30 wherein hot air from the inlet 21 is blown through the sheet 18. The spent hot air is withdrawn at the outlet 31. The temperature of the hot air for this drying step is initially controlled such that it is in the range of from about 200 F. to about 300 F. as drying is completed. This temperature is usually somewhat lower than the temperature used for normal hot gas impingement. It is preferably lower since the foam sheet contains less water and thus is more susceptible to heat damage. In some cases it is possible to remove all of the remaining moisture by phase change using only the step of blow through drying rather than the combination of impingement drying plus blow through drying. The selection of the preferred step or steps would depend on factors such as desired speed of drying, nature of the foam, density of the foam, structure of the foam, etc.

The drying steps described above can be used in various combinations to accomplish the objects of the invention. The complete combination, including foam pre-heating followed by free gravity and forced drainage, plus the phase change sequence of normal hot gas impingement, plus hot gas blow through, would be preferred in some cases, particularly when the fastest possible drying of foam is desired. The nature and density of the foam material may at times make it feasible to eliminate the step of foam pre-heating. Similarly, drainage may be carried out with free gravity drainage alone or with a combination of free gravity drainage and forced drainage, e.g., vacuum removal of water. A phase change step is essential to complete drying. In the practice of the present invention this may comprise hot gas impingement alone or it may comprise hot gas blow through alone, or a combination of impingement and blow through. In any case, the preferred sequence can be determined for a given foam drying sequence depending on the desired results in order to achieve an improved method over those previously known which are limited to combination of drainage without foam preheating plus phase change by parallel flow drying.

The following specific examplesare offered in illustrations of the practice of the herein described improved method:

EXAMPLE 1 One hundred parts of a slurry of demineralized water containing 4.35 parts by Weight cotton cellulose linters refined to a Canadian Standard Freeness of 400 ml. is admixed with 1.43 parts by weight of a 10% solution of commercial grade straight chain sodium alkyl benzene sulfonate having an approximatemolecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The cotton linters are refined type 1AR500 drylap of the type made by Buckeye Cellulose Corporation of Memphis,

6 Tenn. as described in their technical bulletin number 1008 published in 1956.

Air is whipped into the above mixture to produce a foam which has a volume 3% times greater than that of the slurry before foaming. The foam is heated to a temperature of F. and deposited as a sheet 2" thick and 48" wide on the wire of a Fourdrinier which is moving through an enclosed zone having an atmosphere in which the wet bulb temperature is controlled at F.

The wet foam deposited on the screen contains 60 lbs. water per pound solid. This is drained to reduce the moisture content to 30 lbs. water per pound solids. A vacuum of 2" water is then applied through the wire to withdraw additional moisture from the foam until it is dried to 13 pounds water per pound solids.

The foam sheet is then moved into a blower chamber where hot air at 350 F. is impinged against the opposite surfaces of the sheet until the moisture is reduced to 7 pounds water per pound solids. Hot air at 250 F. is then blown through the sheet to remove the remaining moisture. The finished product is a dry, absorbent, fibrous, sponge-like material having a density of 2 pounds per cubic foot.

EXAMPLE 2 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight southern slash pine Kraft wood pulp refined to a Canadian Standard Freeness of 400 ml. and 0.1 part by weight Kymene 557 (as manufactured by the Hercules Powder Company of Wilming ton, Del., and described in their 1961 technical data bulletin PM507) solids is admixed with 1.00 part by Weight of a 10% solution ofv commercial grade straight chain sodium alkyl benzene sulfonate having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The wood pulp is type HP-ll drylap of the type made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin number 1033 published in 1962.

Air is whipped into the above mixture to produce a foam having a volume 1.85 times that of the mixture before foaming. The foam is heated to a temperature of 120 F. and deposited as a sheet 1" thick and 48" wide on the wire of a Fourdrinier which is moving through an enclosed zone having an atmosphere in which the wet bulb temperature is controlled at 140 F.

The wet foam deposited on the screen contains about 65 lbs. water per pound solid. This is drained to reduce the moisture content to 20 lbs. of water per pound solids. A vacuum of 2" water is then applied through the wire to withdraw additional moisture from the foam until it is dried to 18 pounds water per pound solids. A screen surface is pressed down onto the top foam surface to press out additional water and reduce the moisture content to about 15 pounds water per pound solids and to impart a paper-like top skin to the finished pad.

The foam sheet is then moved into a blower chamber where hot air at 400 F. is impinged at a velocity of 3500 feet per minute against the opposite surfaces of the sheet. The velocity of impingement is increased to 11,000 feet per minute until the moisture is reduced to about 4 pounds water per pound solids. Hot air at 250 F. is then blown through the sheet to remove the remaining moisture. The finished product is a dry, absorbent, fibrous, sponge-like material having a density of 1.6 pounds per cubic foot.

EXAMPLE 3 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight cellulose cotton linters refined to a Canadian Standard Freeness of 450 ml., 0.2 part by weight of 26% solution of melamine formaldehyde resin and 0.7 part by weight of a 10% solution of 40-60 styrene-butadiene latex, is admixed with 2.3 parts by weight of a 5% solution of commercial grade straight chain sodium alkyl benzene sulfonate having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The cotton linters are type 1AR500 made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin No. 1008 published in 1956.

Air is whipped into the above mixture to produce a foam having a volume four times that of the mixture before foaming. The foamed mixture containing about 60 pounds water per pound dry solids is then deposited as a sheet A" thick and 48" wide on a moving Fourdrinier wire, and is drained by a combination of free and vacuum-assisted drainage until the moisture content reaches 13 pounds water per pound dry solids. This drainage requires 2 minutes.

The foam sheet then passes into an enclosed chamber where hot air is impinged against both surfaces. First, air at 350 F. is impinged at 2500 feet per minute for one minute, reducing the moisture content from 13 to 8.5 pounds water per pound dry solids. Then air at 325 F. is impinged at 3800 feet per minute for one minute, reducing the moisture content to 3.5 pounds water per pound dry solids. Finally, air at 300 F. is impinged at 7200 feet per minute for one minute to remove the remaining moisture. The total time required to reduce the moisture content from 60 pounds water per pound dry solids to dryness is five minutes. The finished product is a light fibrous material having a density of 0.6 pound per cubic foot.

EXAMPLE 4 The process of Example 3 is carried out with the same material except that the foam is heated to 120 F. prior to deposition on the moving Fourdrinier wire. The foam,

having a moisture content of 60 pounds water per pound dry solids, is drained by a combination of free and vacuum-assisted drainage until the moisture content reaches 13 pounds water per pound dry solids. This drainage requires 1.5 minutes. The process is completed as in Example 3. The total time required to reduce the moisture content from 60 pounds water per pound dry solids to dryness is 4.5 minutes.

EXAMPLE 5 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight southern slash pine Kraft wood pulp refined to a Canadian Standard Freeness of 400 ml. and 0.1 part by weight polyamide wet strength resin solids is admixed with 1.00 part by Weight of a solution of commercial grade straight chain sodium alkyl benzene sulfonate having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The wood pulp is type HP-ll drylap of the type made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin No. 1033 published in 1962.

Air is whipped into the above mixture to produce a foam having a volume 1.85 times that of the mixture before foaming. The foam is deposited as a sheet 1" thick and 48" wide on the wire of a Fourdrinier which is moving through an enclosed zone having an atmosphere in which the wet bulb temperature is controlled at 140 F.

The wet foam deposited on the screen contains about 65 lbs. water per pound solid. This is drained by a combination of free and vacuum-assisted drainage until the moisture content reaches 15 pounds of water per pound solids, This drainage requires 6 minutes.

The foam sheet is then moved into a blower chamber where hot air at 300 F. is impinged against the foam sheet for 7 minutes at a velocity of 3500 feet per minute. This reduces the moisture content from 15 to 5 pounds water per pound dry solids. Hot air at 300 F. is then blown through the material against a pressure drop of 15" water. The remaining moisture in the sheet is re- 8 moved in 4 /2 minutes. The total time required to reduce the moisture content from 65 pounds water per pound dry solids to dryness is 17 /2 minutes. The finished product is a dry, absorbent, fibrous, sponge-like material having a density of 1.6 pounds per cubic foot.

EXAMPLE 6 The process of Example 5 is carried out with the same material except that the foam is heated to F. prior to deposition on the moving Fourdrinier wire. The foam, having a moisture content of 65 pounds water per pound dry solids, is drained by a combination of free and vacuum-assisted drainage until the moisture content reaches 15 pounds water per pound dry solids. The drainage requires 5 minutes. The process is completed as in Example 5. The total time required to reduce the moisture content from 65 pounds water per pound dry solids to dryness is 16 /2 minutes.

EXAMPLE 7 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight southern slash pine Kraft wood pulp refined to a Canadian Standard Freeness of 400 ml. and 0.1 part by weight polyamide wet strength resin solids is admixed with 1.00 part by weight of a 10% solution of commercial grade straight chain sodium alkyl benzene sulfonate having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The wood pulp is type HP-ll drylap of the type made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin No, 1033 published in 1962.

Air is whipped into the above mixture to produce a foam having a volume 1.85 times that of the mixture before foaming. The foam is deposited as a sheet 1" thick and 48" wide on the wire of a Fourdrinier which is moving through an enclosed zone having an atmosphere in which the wet bulb temperature is controlled at F.

The Wet foam deposited on the screen contains about 65 pounds water per pound solid. This is drained to reduce the moisture content to .20 pounds of water per pound solids. A vacuum of 2" water is then applied through the wire to withdraw additional moisture from the foam until it is dried to 18 pounds water per pound solids. A screen surface is pressed down onto the top foam surface to press out additional water and reduce the moisture content to about 15 pounds water per pound solids and to impart a paperlike top skin to the finished pad. The time required to reduce the moisture content from 65 to 15 pounds water per pound solids is six minutes.

The foam sheet is then moved into a blower chamber where hot air is impinged against the opposite surfaces of the sheet. First, air at 250 F. is impinged at 3500 feet per minute for eight minutes, reducing the moisture content from 15 to 4 pounds water per pound dry solids. Then air at 250 F. is impinged at 8,000 feet per minute for 2 minutes reducing the moisture content to 2.5 pounds water per pound dry solids. Finally, air is impinged at 11,000 feet per minute, first at a temperature of 250 F. for 10 minutes and then at 200 F. for 4 minutes to remove the remaining moisture.

The total time required to reduce the moisture content from 65 pounds water per pound dry solids to dryness is 30 minutes. The finished product is a dry, absorbent, fibrous, sponge-like material having a density of 1.6 pounds per cubic foot.

EXAMPLE 8 The process of Example 7 is carried out with the same material, except that the foam is heated to 120 F. prior to deposition on the moving Fourdrinier wire. The foam having a moisture content of 65 pounds water per pound solids, is gravity drained, vacuumed, and pressed in the same manner as in Example 7. The drainage requires 4 minutes. The drying process is completed as in Example 7. The total time required to reduce the moisture content from 65 pounds water per pound dry solids to dryness is 29 minutes.

EXAMPLE 9 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight southern slash pine Kraft wood pulp refined to a Canadian Standard Freeness of 400 ml., 1.2 parts by weight of a 10% solution of polyamide wet strength resin, and 8.7 parts by weight of a 10% solution of 4060 styrene-butadiene latex is admixed with 2.4 parts by weight of a solution of commercial grade straight chain sodium alkyl benzene sulfonate, having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The pulp is of type HP-11 drylap made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin number 1033 published in 1962.

Air is whipped into the above mixture to produce a foam having a volume thre times that of the mixture before foaming. The foamed mixture containing 43 pounds water per pound dry solids is then deposited as a sheet 21 thick and 48" wide on a moving Fourdrinier wire and is permitted to drain freely until the moisture content reaches 21.5 pounds water per pound dry solids. A vacuum of 2" water is then applied through the wire to reduce the moisture content to 7.4 pounds water per pound dry solids. The time required to reduce the moisture content from 43 to 7.4 pounds water per pound dry solids is 2% minutes.

The foam sheet then passes into an enclosed chamber where air at 300 F. is blown through the foam material against a pressure drop of 12.8" water. The remaining moisture in the sheet is removed in /2 minute. The total time required to reduce the moisture content from 43 pounds water per pound dry solids to dryness is three minutes. The finished product is a very thin, coarse, fibrous material having a density of 11.4 lb./ft.

EXAMPLE The process of Example 9 is carried out with the same material, except the foam is heated to 120 F. prior to deposition on the moving Fourdrinier wire. The foam, having a moisture content of 43 pounds water per pound dry solids, is drained and vacuumed in the same manner as in Example 9. The drainage requires 1% minutes. The drying process is completed as in Example 9. The total time required to reduce the moisture content from 43 pounds water per pound dry solids to dryness is 2% minutes.

EXAMPLE 11 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight cellulose cotton linters refined to a Canadian Standard Freeness of 450 ml., 0.2 part by weight of 26% solution of melamine formaldehyde resin and 0.7 part by weight of a 10% solution of 40-60 styrene-butadiene latex, is admixed with 2.3 parts by weight of a 5% solution of commercial grade straight chain sodium alkyl benzene sulfonate having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The cotton linters are type 1AR500 made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin number 1008 published in 1956.

Air is whipped into the above mixture to produc a foam having a volume four times that of the mixture before foaming. The foamed mixture containing 60 pounds water per pound dry solids is then deposited as a sheet A" thick and 48" wide on a moving Fourdrinier wire, and is drained by a combination of free and vacuum-assisted drainage until the moisture content reaches 15 pounds water per pound dry solids. This drainage requires 2 minutes.

The foam sheet then passes into an enclosed chamber where air at 300 F. is impinged at 2500 feet per minute against the top surface. The moisture is removed after 6 /2 minutes of impingement. The total time required to reduce the moisture content from 60 pounds water per pound dry solids to dryness is 8 /2 minutes. The finished product is a light, fibrous material having a density of 0.6 pound per cubic foot.

EXAMPLE 12 One hundred parts of a slurry of demineralized water containing 1.5 parts by weight cellulose cotton linters refined to a Canadian Standard Freeness of 450 ml., 0.2 part by weight of 26% solution of melamine formaldehyde resin and 0.7 part by weight of a 10% solution of 40-60 styrene-butadiene latex, is admixed with 2.3 parts by weight of a 5% solution of commercial grade straight chain sodium alkyl benzene sulfonate having an approximate molecular weight of 342 and in which the alkyl chain length is in the range of from about C to about C and averaging C The cotton linters are type 1AR500 made by Buckeye Cellulose Corporation of Memphis, Tenn. as described in their technical bulletin number 1008 published in 1956.

Air is whipped into the above mixture to produce a foam having a volume four times that of the mixture before foaming. The foamed mixture containing 60 pounds Water per pound dry solids is then deposited as a sheet A" thick and 48" wide on a moving Fourdrinier wire, and is drained by a combination of free and vacuum-assisted drainage until the moisture content reaches 15 pounds water per pound dry solids. This drainage requires 2 minutes.

The foam sheet then passes into an enclosed chamber where air at 300 F. is impinged at 2500 feet per minute against th bottom surface. The moisture is removed after 7 /2 minutes of impingement. The total time required to reduce the moisture content from 60 pounds water per pound dry solids to dryness is 9 /2 minutes. The finished product is a light, fibrous material having a density of 0.6 pound per cubic foot.

While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention and it is intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.

What is claimed as new is:

1. A method of drying a wet foam containing randomly suspended fibers to produce a dry fibrous spongelike product of low density comprising the steps of:

(a) 2laeating the wet foam to a temperature of about (b) depositing the heated foam as a sheet of uniform thickness on a foraminous support,

(c) draining said sheet to remove from about 75% to about of the water in said foam,

(d) removing the remaining water in said sheet by phase change whereby said water is vaporized.

2. A method of drying a wet foam as claimed in claim 1 wherein removing the remaining water by phase change comprises impinging a hot gas normal to at least one surface of the foam sheet.

3. A method of drying a wet foam as claimed in claim 1 wherein removing the remaining water by phase change comprises blowing a hot gas through the foam sheet.

4. A method of drying a wet foam as claimed in claim 1 wherein removing the remaining water by phase change includes the steps of:

(a) impinging a hot gas normal to at least one surface of the foam sheet until the water content of the foam sheet has been reduced to a range of from 3,542,640- 11 I E r v 12 about 1.5% to about 15% of the original water con- OTHER REFERENCES tent of the foam sheet and SIZIIlg and the Tem rature of the Stock, 1n TAPPI (b) blowmg a hot gas through the foam sheet to re- Special Reports, NO. 2 1 (1926) move the remamlng water.

References Cited v 5' s. LEON BASHORE, Primary Examiner UNITED STATES PATENTS R. H. TUSHIN, Assistant Examiner 1,870,279 8/1932 Bryant 162--101 3,263,336 8/1966 Sjogren 34 23 US. Cl. X.R. 3,303,576 2/1967 Sisson 34 115 10 34 23; 162-207

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