METHOD FOR PREPARING MODIFIED CELLULOSIC FIBERS

Abstract: Disclosed is a method for preparing modified cellulosic fibers comprising reacting linear, water-wettable polysacch­ arides with N,N′-methylenebisacrylamide under special con­ ditions. The materials are useful in the preparation of prod­ ucts characterized by their increased bulk and absorbency.

Description: METHOD FOR PREPARING MODIFIED CELLULOSIC FIBERS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to the modification of polysaccharide materials by treatment with an amide. When such polysaccharide materials are cellulosic fibers, webs comprising such modified fibers and untreated fibers exhibit increased bulk and absorbency. DEFINITIONS For purposes of this invention, a "water-wettable polysaccharide" is one which is either insoluble in water or capable of absorbing water and being swollen thereby. As used herein, the term "cellulosic fibers" refers to fibers comprising cellulose, a linear, water-wettable polysaccharide, whether existing as a single constituent in a larger natural aggregate such as wood pulp, bagasse and cotton linters, or as a derivative of the natural aggregate such as alpha pulp or viscose rayon. The term "fines" means cellulosic fibers less than 2 mm in length. -The- term "consistency"- means the weight of fibers in a pulp suspension or wetted fibrous batt or web usually expressed as a percentage. For example, ten pounds oven dry fibers in one hundred pounds of a mixture of water and fibers would be a suspension of 10% consistency. SUMMARY OF THE INVENTION The present invention relates to the preparation of novel cellulosic fibers comprising the reaction product of linear, waterwettable polysaccharide fibers with N,N'-methylenebisacrylamide. The reaction is carried out by combining the N,N'-methylenebisacrylamide in an aqueous alkaline medium having a pH between 8.5 and 13.5 with an aqueous slurry or mixture of the cellulosic fibers or by applying it in the form of a solution to a batt or web of cellulosic fibers. The consistency of said slurry or mixture should be at least 10% in order to effect a satisfactory efficiency of the reaction. Optimally, slurries or mixtures are of 20-45% consistency. Batts or webs are generally of greater than 45% consistency. By following the teachings of the present invention, efficiencies, (as measured by nitrogen assays of washed fibers) of from 40% to greater than 70% can be obtained. The reaction may be allowed to proceed for several days at ambient temperature, for example 250C, or can be brought to completion within hours or minutes at elevated temperatures provided that the temperature of the aqueous medium remains below its boiling point. Free radical initiators should be excluded from the reaction medium, because they neutralize the reactive sites on N,N'-methylenebisacrylamide for carbamoylethylation, namely, the double bonds. Air can likewise have a poisoning effect on the desired reaction. After the N,N'-methylenebisacrylamide and cellulosic fibers are combined, it is necessary to exclude air from the container for the materials where the reaction is to take place. In order for the desired reaction to proceed quiescent, non-aerated conditions which do not permit the introduction of air, should be employed. While obviously some air is generally dissolved in water and may be entrapped in the interstices of a batt of fibers, if the reactants are freely exposed to ambient air, virtually no N, N '-methylenebisacrylamide will be found to have reacted with the cellulosic fibers. It will therefore be apparent that conditions or actions which cause air to be entrapped or entrained in the medium, such as application by spraying or permitting the medium to boil, should be avoided. DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the method of the present invention, the reaction is carried out in an aqueous alkaline medium having a pH between 8.5 and 13.5 and preferably between 11 and 13, using either an aqueous slurry or mixture of cellulosic fibers or by applying the N, N'-methylenebisacrylamide in solution to a batt or web of cellu losic fibers, for example pulp lap. # The amount of N, N'-methylene- bisacrylamide to be dissolvéd in the aqueuse alkaine meditium yvith the cellulosic fibrous material should be suffieient to imlparit the desired increase in bulk and absorbency Above about 25% N N' methylenebisacryliamidde by dry Weight of the fib there no additional effect on the fibers. Insufficient N' N' thy bis acrylamide generally below 0.2% will produce no perceptible change in the bulk or total vater absorption of aroveb of the fibers although any measurable amount of N, N-methy lenebisadfylamide pro duces a measurable increase in absorbency@ ie. the ate of absorption. In practice using sufficienti N N' - methylenebisacry-larnide to cause from about 0.5 to 2% by dry weigh, t2 of the fibers to react with the fibers produces a level of modification su fficient for commercial applications. In accordance with an, altern@tive ernbodinen of the pnesent invention, when it is desired to cary the rects at very high consistencies, for example above bO%., theaqueous alka line medium (in which the N, N - methylenebisadry de is soluble may be inco@ porated into a water miscible organic" solv@@@t such as acetone This " combination provides a greavervoume in which to slufry or immerse the fibers and with which to proteet the fiber from air There fore, as used here the term "adueous alk@line medium" comprchends mixtures of water with water - inisciblei organic so vents which do not interfere with the reaction betveer In N' in thylenebisacr lamide and the cellubsic fiber s. The novel modified fibers. of this in vention mav be used in com bination vith Gonventional pap aking fibers to producewebs which exhibit increased buk and abson bency. The modified fibers of the present invention dimp dve be bulk and absor bensy of the base. web in direct proportion to theo pascentagei of modified fiber in- the blend. Alter natively obending can permit a reduction in. basis weght while retaihing bulk and bso bency By way of illustration, in ad web contining 30 modified fiber basis weight was reduced by 25% without loss of bulk and absorbency as compared with a web without modified fibers of the present invention. An other utility of the fibers of the invention is as a replacement for any of the kmovn super absorbent, fibers such as the "super slurper" fibers. for use ind a vardety ofabsorbest products such as diapers sanitary napkin, hos it dressings and the like Unlike the typical super-absor bent fiber@@ which characteristically have a gelatinous or "slimy' fo when wet the fibens made in accordance with the plesent chtion retain their wogd pulp-like feel to the touch. A fur ther advant@ge bf. the present invention is that the modified fibers exhibit. wet resüence That is to say, the fibers retain their bulk when wet which is important in many absorbent products, such as diapers wiberevt be shape and volume of the product when wet plays an importand part ihn the function of the product e.g. in retaining fit and wicking. Previous inventors have attempted to achieve thes same properies but found it necessary to employ rather elaborate and severe chemical conditions to achieve only some of these properties. Of ex ple Lask, U S patent 4,248, 595 granted February 1981 isckoses a methodto produce "swellable carboxy kylce @ ses" f@@@ dwe-part, chemical system comprising first a carboxyalky the fying agent and crosslihking agent in @@@i @@ ou @@@ @@@@@e nedium Furthermore, a very much larger propor@on f@@be" e@@ @@@ng agent and alkali based on the weight f@fiber is dre@uired by the precess Lask relative to N, N' @@thy lenerbis @r@@ aride which is emoloyed by Lask selely to crosslink the etherifyin g agent. In the prqsent invention. no such cther@ying age is @ p@yed diu @@ @@@er kal co@cen- tration is qu@ed and the e@@i@@ che@ isty of N N' methylene bisacrylamide resembles T of @ @@@@@ @@@@@ @@@ion lass of reactions as opposed t eress @@@ing@@ d @@@bed by lask The present invento @@@@@@ of @@ scles@@@ in @@@@ art which would sugges such @@@@@ frond N, N' @@@@@ @@@ amide. indeed, several @@@@ des @@@@ @@@@ wers tried but die not produce clear impravemes @@@ abso@bency and b@@k These compounds include Acrylamide N-Methylolacrylamide Bisacrylamide Glyoxal N-lsopropylacrylamide N-N Dimethylamino ethyl methacrylate Triallylcyanurate Glycidyl Acrylate Acrylic Acid The remarkable results attained with N,N'-methylenebisacrylamide in accordance with the present invention are especially surprising in view of the prior art. The alkali concentrations employed in the present invention are relatively low. In such alkali media of relatively low concentration, the base catalyzed reaction of cellulose with acrylamide has long been believed to produce carbamoylethylcellulose. U.S. Patent 2,338,681 granted to Bock et al January 4, 1944. There is no suggestion in the prior art that carbamoylethylcellulose provides greater bulk and absorbency to webs. Indeed, the pertinent prior art suggests that such forms of cellulose are soluble in water when sufficiently modified with acrylamide. Bikales et al, U.S. Patent 3,029,232 granted April 10, 1962. Bikales et al observed in Example 8 that paper made from cellulose treated with N-n-propylacrylamide is stronger than from untreated pulp. However, they reported no change in absorbency of the fibers. Applicant's fibers, in contrast, form weaker webs having greater absorbency and bulk. It is to be concluded, therefore, that the reaction product of the present invention has a chemical composition different from or comprises something in addition to carbamoylethylated cellulose as understood by the prior art. Treatment of wood pulp fibers treated in accordance with the present invention results in extended hydrophilicity, increased brightness and receptivity to ink, and in fibers which are more readily debonded thus resulting in bulkier, softer paper than the same fiber in the untreated form. There is at the same time no perceptible change in the structural appearance of the fibers. These improved properties are retained by the fibers when subjected to typical stress to which pulp fibers are subjected, for example, boiling water, moderately strong acid and alkali, and bleaching chemicals. Bleaching with chlorine, chlorine dioxide, hydrogen peroxide, ozone and combinations of such bleaching steps will not undo the fiber modification. This persistence of the structural integrity of the fibers (as well as the improved properties) further distinguishes the fibers resulting from the process of the present invention from those carbamoylethylated cellulose fibers of the prior art which are soluble in water. See U.S. patent 3,029,232, Column 5, lines 14-15. The method of the present invention is not limited to any particular type of cellulosic fiber and has been successfully employed on a wide variety of wood pulps, both chemical and mechanical, hardwood and softwood, bagasse, secondary (recovered waste paper) and rayon staple fibers. In one embodiment of the present invention, the N,N'-methylenebisacrylamide reagent is combined with the sodium hydroxide used for the second extraction stage during a bleaching sequence such as CEHED, chlorine-alkali extraction hypochlorite-alkali extraction - chlorine dioxide or CEDED, chlorine-alkali extraction - chlorine dioxide-alkali extraction - chlorine dioxide. The temperature e.g. 600C, and duration, typically one hour, of such a stage are sufficient for the modification reaction to be completed. The final bleach stage (chlorine dioxide) serves as a neutralization and washing step. The present invention provides overall improvement in the wood pulps produced by "high yield" or mechanical pulping processes, e.g. thermomechanical and refiner mechanical pulps, which are characterized in having larger hemicellulose and carbohydrate contents than chemical pulp. Since, as previously mentioned, the modification imparted to cellulosic fibers by N ,N '-methylenebisacrylamide survives delignification treatments, the aforementioned treatment of "high yield" or mechanical pulps can be advantageously combined with a delignification step, either prior to or subsequent to application of the method of the present invention. Mechanical pulps are desirable in that they are produced in high yield, but have found limited use in absorbent paper products due to their rigid structure. In the past, attempts have been made to produce fibers which have enhanced flexibility compared to groundwood, refiner mechanical pulp, thermomechanical pulp, chemirefiner and chemi-thermomechanical pulp though delignification. Unfortunately, total or partial delignification produces pulps with reduced bulk. The latter phenomenon is disadvantageous when the end-use of the fiber is in absorbent products. In accordance with the present invention mechanical fibers can be made flexible while maintaining or improving their bulk characteristics. These fibers are subjected to, for example, ozonization whereby at least partial delignification is achieved, resulting in low bulk high-bonding fibers, which upon subsequent treatment in accordance with the present invention results in a pulp with high brightness, bulk and flexibility. Alternatively, the method of the present invention can be applied prior to the delignification stage. In this connection, it is to be noted that fibers treated in accordance with the present invention exhibit sharp reductions in bleach chemical demand. A further advantage of the present invention is that when short cellulosic fibers, hereinafter called fines, are modified in accordance with the present invention, they become non-bonding and dispersible. This feature has significant economic implications. In particular, it permits the use of pulp furnishes containing a high proportion of fines without the normal difficulties. Wood fines, an assortment of particulate wood products which pass through a 75 micron opening, exhibit rather noticeable adhesive properties uncom mon to regular wood fibers. Consequently, when isolated and dried they form a dense agglomerated structure which resists being dis persed in water. In this agglomerated state, fines interfere with both the manufacture of absorbent papers and their product quali ties. When fines are treated in accordance with the present inven tion, they become soft and dispersible in water after drying. By way of illustration, a stone ground wood pulp containing 238 fines, when formed into a mat from an aqueous dispersion, dried into a rough textured non-dispersible mass. When the same pulp was treated in accordance with the method of present invention with 2.5% N,N'-methylenebisacrylamide based on dry pulp weight, neutralized and dried, the treated fines were found to be soft and dispersible. It can be appreciated by one of ordinary skill in the art to which the present invention pertains that a large number of variations may be effected in reacting the cellulosic fibrous material with N, N '-methylenebisacrylamide in accordance with the reaction procedures described above, without materially departing from the scope and spirit of the invention. The following examples will more fully illustrate the embodiments of this invention. In the examples all temperatures are in degrees Celsius. "Wet pick-up" is expressed as a percent by weight of the dry fibers to which the solution is applied. The abbreviation "TWA" stands for "total water absorbed" and is determined on a gram for gram basis, e.g. a TWA of 2 means 2 grams of water were absorbed for each gram of fiber. Tensile measurements were obtained on a Thwing Albert Tensile Tester in accordance with TAPPI Standard Number T 456m-49. Tensile was measured cross direction (CD) and machine direction (MD) for a dry strip. All tensile values are reported as ounces/inch. These values may be converted to the standard metric unit of grams per 15 millimeters by multiplying by 16.775. EXAMPLE 1 A roll of paper having a basis weight of 30 grams per square meter and made from northern softwood kraft pulp was saturated by means of a gravure apparatus to the extent of 74% wet pick-up with a solution comprising 6% by weight N,N'-methylenebisacrylamide and 5% potassium hydroxide. The resulting impregnated web had a consistency of 58%. After a 14-day reaction period at room temperature in sealed plastic wrap, specimens were withdrawn for evaluation. The results are shown in Table 1. Sample A is a dry sheet made from the same lot of northern softwood kraft pulp as was treated in this Example. Sample B represents Sample A after refin ing in a Valley beater to increase its breaking length. Sample C represents the modified pulp described in this Example. Sample D is a sheet made from a dispersion of the sheets of Samples B and C mixed in equal proportions, formed into a web and dried. EXAMPLE 2 Modified fiber in accordance with the present invention was produced by feeding northern softwood kraft pulp saturated to the extent of 3308 wet pick-up with a solution comprising 2.9% N,N'methylenebisacrylamide and 1 . 4% potassium hydroxide into a steamjacketed high consistency continuous refiner mixing device heated to 800C where it resided for two minutes. The reacted pulp was collected at a consistency of 28%, diluted and acidified with phosphoric acid to pH 7 and transferred to a paper machine where the modified pulp was blended with untreated northern softwood kraft in the proportion of 50% by dry weight to all the fibers to produce towel 2 weight webs of 41g/m . The results are presented in Table 2. TABLE 1 -------- Braking % MBA Bulk in Basis Wgt Legth* TWA Basis Composite gm/m2 Meters gm/gm Weight ---------- ------------- --------- --------- ------- A. Control 0 44.0 2621 2.84 4.8 B. Control Refind 0 39.0 9040 1.92 4.3 C. Modified Fiber 1.93 41.0 945 5.40 7.1 D. B/C = 1/1 Blend 0.97 44.0 4114 3.72 5.2 * "Breaking length" is the estimated length at which the web would break under its own weight. It is derived from a measurement of the tenslle strength of the web and related to its basis weight. While non-empirical, it is useful in comparing webs of different] weights. TABLE 2 % Modified Fiber in Blend 0 40 Pulp Freeness 680 709 (Canadian Standard) Basis Weight g/m2 40.0 41.0 Assayed N, N '-methylenebisacrylamide 0 .66 % by weight of dry fiber in web Physical Properties 24-ply bulk 4.6 4.9 TWA (g/g) 3.7 5.6 Stretch (percent) Machine Direction 12.0 11.5 Cross Direction 1.7 2.7 Dry Tensile Machine Direction 38.9 19. 5 Cross Direction 30.3 16.5 EXAMPLE 3 Sheets of dry lap pulp (southern softwood kraft weighing 800 2 on an air dry basis) were uniformly impregnated with a solugim tion at a temperature of 500 comprising 2.596 N,N'-methylenebisacrylamide and sufficient sodium hydroxide to achieve a pH of 11.5 using a gravure type applicator so as to provide a wet pick-up of 56% and a consistency of 64%. The impregnated pulp was rolled-up, enclosed in plastic film and allowed to react at room temperature for 30 days before quenching to pH 7.0 with a very dilute solution of aqueous phosphoric acid. The modified pulp contained 1.57% by weight N, N'-methylenebisacrylamide 56% of the amount applied based on nitrogen assays. The modified pulp was blended with untreated, beaten southern softwood pulp in the proportion of 35% by dry weight to all the 2 fibers to produce towel weight webs of approximately 50 and 40gum The results are presented in Table 3 in which "basis weight" is abbreviated B.W. and "breaking length" is B.L. and "absorbency" is ABS. The degree of refining (REFINE) by means of a Valley beater, is represented in minutes. Refining of the samples with modified pulp was used to achieve a breaking length comparable to that of the control. As may be seen, a web of comparable TWA and bulk can be achieved at a much lower basis weight by incorporation of the modified pulp of the present invention. TABLE 3 CONTROL WEB AT HICH BASIS WEICHT REFINE B.W. BULK DENSITY BULK MOT CDT B.L. TWA ABS (min) g/mê mm g/cc B.W. oz/in oz/in m g/g secs. 5 53.2 5.28 0.516 6.61 53.40 37.67 938.89 2.9 22.5 MODIFIED PULP SUBSTITUTION IN WEB HAVING LESSER BASIS WETGBT REFINE B.W. BULK DENSITY BULK WDT CDT B.L. TWA ABS. (min) g/mê mm g/cc B.W. oz/in oz/in m g/g secs. 7 49.5 5.59 0.437 7.54 32.1 21.9 595.67 3.7 6.9 15 43.1 4.64 0.531 7.20 46.0 29.2 950.78 2.9 30.5 The absorbent products of the present invention can be used in a variety of applications where absorbency is desired. In particular they are useful in applications such as feminine hygiene products, catamenial devices, disposable diapers and non-wovens for hospital and surgical use. It is apparent that other varianons and modifications may be made without departing from the present invention. Accordingly, it should be understood th t the for s of the present invention de Cu scribed above are @@ust ative on@ nd not iilded to limit the scope of the invention as defined byathe appended claims

Claims: WHAT IS CLAIMED IS: 1. A method for preparing modified cellulosic fibers which comprises the steps of reacting cellulosic fibers with N,N'-methylenebisacrylamide by dry weight of said fibers in an aqueous alkaline medium having a pH between 8.5 and 13.5 and a consistency of at least 1096 while excluding air and maintaining the temperature of the aqueous medium below its boiling point. 2. The method of claim 1 wherein said fibers are in the form of a slurry or mixture. 3. The method of claim 2 wherein the consistency is between 20 and 45%. 4. The method of claim 1 wherein said fibers are in the form of a batt or web. 5. The method of claim 4 wherein the consistency is greater than 45%. 6. The method of claim 1 wherein said fibers are fines. 7. The method of claim 1 wherein said fibers are wood pulp fibers derived from a mechanical pulping process. 8 A process for delignifying pulp wherein the delignification stage is preceded by the method of claim 1.

Inventors: BOX, LARRY
Applicants: SCOTT PAPER COMPANY
Application year: 1986
Main classification: C08B15
IPC codes: D21H11, D06M13, D06M101, C08B15, D21H13, D06M14, A61L15