|1.||ED I S : Modified polysaccharide materials comprising a waterwettable polysaccharide reacted with up to 25% of N , N'methylenebisacrylamide by dry weight of said polysaccharide.|
|2.||Materials according to claim 1 wherein said polysaccharide is in the form of a powder.|
|3.||Materials according to claim 2 wherein said polysaccharide is corn starch .|
|4.||Materials according to claim 1 wherein said polysaccharide is in the form of cellulosic fibers .|
|5.||Materials according to claim 4 wherein said fibers are fines .|
|6.||" Fibers according to claim 4 reacted with 0.5 to 2% N , N' methylenebisacrylamide by dry weight of the fibers .|
|7.||A method for preparing modified polysaccharide materials which comprises the steps of reacting a waterwettable poly saccharide with up to 25% of N , N'methylenebisacrylamide by dry weight of said poly¬ saccharide in an aqueous alkaline medium having a pH between 8.5 and 13.5.|
|8.||8 The method of claim 7 wherein said polysaccharide is in the form of cellulosic fibers .|
|9.||The method of claim 8 wherein the fibers are reacted with 0.5 to 2% N, N'methylenebisacrylamide by dry weight of the fibers .|
|10.||The method of claim 8 wherein the pH is between 11 and 13.|
|11.||The method of claim 8 wherein said reaction is carried out at ambient temperature for two weeks.|
|12.||The method of claim 8 wherein said reaction is carried out at 55°C for six hours.|
|13.||The method of claim 8 wherein said fibers are fines.|
BACKG ROUND OF THE I NVENTION Field of the I nvention 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.
DEFI N ITIONS 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 . These include fibrous cotton and wood pulps, fine-cut cotton and wood pulps, activated polysaccharides such as oxidized cellulose; hydrolyzed polysaccharides such as hydrocelluloses; various types of starch such as corn, potato, wheat starches, as is, or pre-gelatinized; guar gum; and various water-insoluble derivatives of cellulose, starch and other polysaccharides such as carboxymethyl cellulose. As used herein, the term "cellulosic fibers" refers to fibers comprising cellulose, a linear polysaccharide, whether derived from natural materials such as wood pulp, bagasse and cotton linters or synthetic forms such as viscose rayon .
The term "fines" means cellulosic fibers less than 2 mm in length . SUMMARY OF THE I NVENTION
The present invention relates to the preparation of novel poly¬ saccharide materials comprising the reaction product of water- wettable polysaccharide with N, N'-methylenebisacrylamide. The reaction may be 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 of the polysaccharide material or by applying it in the form of a solution to a batt of cellulosic fibers . The reaction may be allowed to proceed for several days at ambient temperature, for example 25°C , or can be brought to completion within hours or minutes at elevated temperatures .
DESCRI PTION OF THE PREFERRED EMBODIMENTS I n 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 of polysaccharide material, such as cellulosic fibers or by applying the N, N'-methylenebisacrylamide to a batt of cellulosic fibers known as pulp lap. Less preferably, the reaction can be carried out in an anhydrous medium using an organic solvent such as acetone, in the presence of alkali . In the case where the water-wettable polysaccharide materials are cellulosic fibers, the amount of N, N'-methylenebisacrylamide to be employed in the reaction with the cellulosic fibrous material should be sufficient to impart the desired increase in bulk and absorbency. Above about 25% N, N'-methylenebisacrylamide by dry weight of the fibers there is no additional effect on the fibers . I nsufficient N Λ N'- methylenebisacrylamide generally below 0.2%, will produce no perceptible change in the bulk or total water absorption of a web of the fibers, although any measurable amount of N, N'-methylenebisacrylamide produces a measurable increase in absorbency i . e. the rate of absorbtion . I n practice from about 0.5 to 2% by dry weight of the fibers produces a level of modification sufficient for commercial applications.
The novel modified fibers of this invention may be used in combination with conventional papermaking fibers to produce webs which exhibit increased bulk and absorbency. The modified fibers of the present invention improve the bulk and absorbency of the base web in direct proportion to the percentage of modified fiber in the blend .
Alternatively, blending can permit a reduction in basis weight while retaining bul k and absorbency. By way of illustration , in a web containing 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 . Another utility of the fibers of the invention is as a replacement for any of the known super absorbent fibers such as the "super slurper" fibers for use in a variety of absorbent products such as diapers, sanitary napkins, hospital dressings and the like. A fu rther advantage of the present invention is that the modified fibers exhibit wet resilence. That is to say, the fibers retain their bul k when wet which is important in many absorbent products, such as diapers, where the shape and volume of the product when wet plays an important part in the function of the product e. g. in retaining fit and wicking.
The present inventor is unaware of any disclosure in the prior art which would suggest such results . I ndeed, several other amides and acrylates were tried but did not produce clear improvements in absorbency and bulk. These compounds include
N- lsopropylacrylamide N-N Dimethylamino ethyl methacrylate
Trial lylcyanu rate
Treatment of cellulosic fibers in accordance with the present invention results in extended hydrophilicity, increased brightness and receptivity to ink, and in fibers which are more readily debonded. 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 . However, successive stages of alkaline treatment such as alkaline extraction - hypochlorination - alkaline extraction will destroy the properties .
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'-methyIenebisacrylamide 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 . 60°C, 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 intensity of overall improvement is greater 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'-methylene- bisacrylamide survives delignification treatments, the afore- mentioned preference for 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. I n the past, attempts have been made to produce fibers which have enhanced flexibility compared to groundwood, refiner mechanical pulp, thermomechanical pulp, chemi-refiner 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 .
I n 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. I n 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 . I n 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 th rough a 75 micron opening, exhibit rather noticeable adhesive properties uncommon to regular wood fibers . Consequently, when isolated and dried they form a dense agglomerated structure which resists being dispersed in water. In this agglomerated state, fines interfere with both the manufacture of absorbent papers
and their product qualities. When fines are treated in accordance with the present invention, they become soft and dispersible in water after drying . By way of illustration, a stone ground wood pulp containing 23% fines, when formed into a mat from an aqueous dispersion, dried into a rough textured non-dispersibie mass .
When the same pulp was treated in accordance with the method of present invention with 2.5% N, N'-methylenebisacryIamide based on dry pulp weight, neutralized and dried, the treated fines were found to be soft and dispersible. When the reaction of the present invention is applied to polysaccharide materials in the form of powders, for example starch , the result is not only increased absorbency but even more surprisingly the property of insolubility. By way of example, cornstarch, a mixture of amylose and amylopectin, which is dispersible in cold water but dissolves in hot water, after reaction with N,N'-methylenebisacrylamide jn accordance with the present invention yields a stable viscous absorbent for cold water, but remains insoluble in either cold or hot water, thus allowing it to become a regenerable absorbent. It can be appreciated by one of ordinary skill in the art to which the present invention pertains that a large number of varia¬ tions 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 TAPP1 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. 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 refining 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 330% wet pick-up with a solution comprising 2.9% N , N'- methylenebisacrylamide and 1 .4% potassium hydroxide into a steam- jacketed high consistency continuous refiner mixing device heated to 80°C where it resided for two minutes . The reacted pulp was collected at a solids content 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
2 pprroodduuccee towel weight webs of 41g/m . The results are presented in
% MBA Basis Wgt Length* TWA Bulk 2 In Composite gm/m Meters gm/g Basis Weight
/ ■ A. Control 44.0 2621 2.84 4.8 i
B . Control Refined 39.0 9040 1 .92 4.3 oo
C. Modified Fiber (3131 -352) 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 tensile strength of the web and related
to its basis weight. While non-empirical , it is useful in comparing webs of different
% Modified Fiber in Blend 0 40
Pulp Freeness 680 709
Basis Weight g/m 2 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
Machine Direction 12.0 11 .5
Cross Direction 1 .7 2.7
Machine Direction 38.9 19.5
Cross Di rection 30.3 16.5
Sheets of dry lap pulp (southern softwood kraft weighing 800 2 g/m on an air dry basis) were uniformly impregnated with a solution at a temperature of 50° comprising 2.5% 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%. 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 fibers to produce towel weight webs of approximately 50 and
2 40g/m . 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 (REFI NE) 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 .
CONTROL WEB AT HIGH BASIS WEIGHT
REFINE B.W. BULK DENSITY BULK MDT CDT B.L. TWA ABS,
(min) g/m mm g/cc B.W. oz/in oz/in m g/g sees
'5 53.2 5.28 0.516 6.61 53.40 37.67 939.89 2.8 22.5
MODIFIED PULP SUBSTITUTION IN WEB HAVING LESSER BASIS WEIGHT
REFINE B.W. BULK DENSITY BULK MDT CDT B.L. TWA ABS
(min) g/m mm g/cc B.W. oz/in oz/in g/g sees
7 49.5 5.59 0.437 7.54 32.1 21.9 595.67 3.7 6.9
T5 43.1 4.64 0.531 7.20 46.0 29.2 950.78 2.9 30.5
Ten-gram specimens of cornstarch were pasted (formed into a paste) with ten milliiiters of a solution containing various proportions of N, N'-methylenebisacrylamide in an aqueous alkaline medium comprising 2% potassium hydroxide, 2% potassium choloride and having a pH between 8.5 and 13.5. The quantity of N, N'-rnethylenebisacryiamide mixed with the cornstarch was in the range of 0.5 to 1 .0% by weight of the cornstarch . Each paste was reacted in a closed Petri dish for 16 hours at 60-70 degrees . The resulting reaction product was a brittle, spongy mass. When dispersed in excess water, neutralized and boiled with minimal venting, this mass swelled to 4 to 6 times its original volume, depending on the amount of N,N'-methylenebisacrylamide applied per weight of starch . The supernatent water was drawn off and the solid dried at 60 degrees.
The resulting product exhibited rapid absorbence of 3 to 6 times its weight in water with commensurate volumetric swelling. Control samples only slowly absorbed less than their own weight in water. Upon boiling in excess water the reaction products disintegrated, rapidly settling as a uniform powder when agitation was stopped . Upon drying the boiled powder returned to its original form and retained its improved absorbency properties. 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 variations and modifications may be made without departing from the present invention . Accordingly, it should be understood that the forms of the present invention described above are illustrative only and not intended to limit the scope of the invention as defined by the appended claims .
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